Several weeks before Aaron Alexis opened fire inside of the Washington Navy Yard on Monday, he was kicked out of a military housing complex in Rhode Island after complaining of too much noise coming from the linen closet, ABC News has learned.

It's unclear if that sparked a series of troubling and bizarre encounters with law enforcement and total strangers in the days that followed.

In the early days of August, Alexis was living at the Naval Bachelor Enlisted Quarters in Newport, R.I., but he complained of hearing noise coming from the linen closet there and was ultimately forced to leave, two sources with knowledge of the investigation told ABC News.

About the same time, Alexis allegedly had a run-in with a family waiting to fly home to Alabama from Norfolk, Va.

On Aug. 4, Glynda Boyd and her family were on their way back from a family reunion when a man she believed was Alexis came out of nowhere inside the airport in Norfolk and accused them of laughing at them.

"He stood in front of us," Boyd told ABC News. "He was no more than two feet away from us, and he would just keep saying, 'Is she laughing at me?" And we were like, 'No.' Then he just got angry. ... You could tell his behavior, something wasn't right with him mentally."

Jacquelyn Martin/AP Photo

An armed officer who said he is with the Department of Defense, warns a vehicle to stay away from the gate at the Washington Navy Yard in Washington, on Sept. 17, 2013

Three days later, Alexis called Newport, R.I., police with a strange story about hearing voices in his head and being under surveillance by shadowy forces after an altercation at a Virginia airport.

The public already knows much of what followed -- including trouble-filled stays at a Marriott in Newport, R.I., visits to a Veterans Affairs medical facility there and then a trek to the Washington, D.C., area.

Once authorities identified Alexis as the suspect in the rampage on Monday, they immediately ran his credit card to look at his past purchases. That search revealed he had rented an AR-15 rifle and then bought the Remington 870 shotgun he brought with him that morning, according to sources. On or about the same day, Alexis also bought a hacksaw from an area Home Depot, the sources said.

The gun and hacksaw purchases could show patterns of planning and speak to Alexis' state of mind, sources said.

Offering the most detailed, official account to date of Monday's massacre, FBI director James Comey told reporters Thursday that the shotgun Alexis used had both its stock and barrel partially sawed off, and he seemed to be "hunting people" with "all different backgrounds" while inside the Navy Yard building. In all, 12 people were fatally shot before Alexis was killed, himself.

Comey didn't say when Alexis first opened fire, but a federal law enforcement official and a local law enforcement official told ABC News he was inside the building for about 10 minutes before first pulling the trigger at about 8:12 a.m on Monday.

Alexis began executing his deadly mission shortly before 8 a.m., when he drove a rental vehicle to a parking deck across from the Navy Yard's Building 197, Comey said.

Wearing cargo pants and carrying a duffel bag, Alexis then crossed the street and, at 8 a.m., entered Building 197 using a valid pass. Inside the duffel bag was the Remington 870 pump-action shotgun, according to Comey.

As a contractor, Alexis had been working on a "server refresh" in Building 197 and had full access to it. On Monday, he first went up to the fourth floor and entered a men's restroom there, Comey said. He then left the restroom and opened fire, shooting people "in no discernible pattern," according to Comey.

After creating initial carnage on the fourth floor, Alexis returned to the first floor, where he fired on a security guard and took his Beretta handgun, Comey said. Alexis then worked his way back to the third and fourth floors, "moving without particular purpose" and gunning down even more victims in the hallway and in their offices, Comey said.

Contrary to earlier reports, it now appears Alexis did not fire into an atrium and cafeteria from a higher floor, according to Comey.

Comey said the first armed responders didn't engage Alexis until 8:30 a.m., nearly 30 minutes after he first entered the building. But, according to law enforcement officials, that would be more than 15 minutes after shots first rang out -- suggesting authorities tracked down Alexis within several minutes of entering the building.

Only days earlier, the head of the Metropolitan Police Department in Washington, Cathy Lanier, said authorities "engaged" Alexis within minutes.

"Within literally two to three minutes [of the first call] Metropolitan Police officers were on the scene," Lanier said. "Now internal security had already ... identified and engaged the suspect. We already had victims down at that point. Within seven minutes we had active shooter teams inside the building moving through the building."

Allegations have surfaced that an elite response team from the U.S. Capitol Police arrived on scene soon after shots first rang out, but they were called back by commanders. A U.S. Capitol Police spokeswoman refused to discuss the substance of the allegations, saying only that her agency "offered and provided mutual support" at the Navy Yard.

A U.S. Capitol Police oversight panel is now looking into the matter, at the request of the agency's chief.

A law enforcement official familiar with the emergency response on Monday, however, said there did not seem to be "a lack of resources" that morning, noting that early on there was still "the possibility that there could be other targets as part of a complex attack."

One more response team at the Navy Yard "probably would not have changed the outcome," the official said.

Nevertheless, a shootout ensued once responders found Alexis inside Building 197.

Comey said investigators have found no indications that others are connected to Monday's attack, and they were still struggling to identify a specific motive.

"There are indications this was a person with mental health troubles, and we're trying to understand that," Comey said.

ABC News has learned Alexis etched the phrases "better off this way" and "my elf weapon" on the stock of the shotgun, which he purchased from a Virginia gun store only days before the attack. Investigators are trying to figure out the meaning of those phrases, especially whether "better off this way" was a signal Alexis expected to be killed by police.

In the days since the attack, details have emerged suggesting Alexis was increasingly becoming paranoid, including recent rantings to police in Newport, R.I., that he was hearing voices in his head and felt microwave machines bombarding his body.

Local police alerted Newport Naval Station authorities about that bizarre story, but the information was never passed up the chain of command, the Navy confirmed Wednesday.

Alexis kept his job as a civilian contractor with the Navy, holding a secret security clearance.